reasonable accommodate

I have lupus & fell & broke my hip at work last August 2003. The fall has exacerbated the illness & my doctor does not want me to work full time any longer. I have inquired about part time, flexible hours, or even transferring to a vacant position. My employer could not accommodate this request & wants me to write a letter of resignation. (they have replaced me) They were kind enough to continue my insurance benefits & have informed me that if I write the letter, any premiums owed will be considered paid. Are they obligated to accommodate my request & how long must they hold my job?

I am a 40+ female; Workers Comp Claim was denied; I do know that there are current positions open within the company & even some that could possibly be done from home. I have worked for the company for 20 years with good reviews.